Amplifier circuits for converting electrical signals into corresponding light signals by applying high-frequency electrical signals to a laser diode are known. For message transmission over long distances, a laser beam can be fed into an optical fiber. In order that the quality of transmission be impaired as little as possible by the conversion of electrical signals into optical ones, errors or distortions occurring during conversion should preferably be kept at a minimum. One way to minimize distortions is to ensure linearity in the conversion.
Circuits for linearizing electro-optical conversions are known. In these circuits a back facet laser signal is converted into a corresponding electrical signal. The corresponding electrical signal is then applied to an input of a differential amplifier; the other input of the differential amplifier is supplied with the nominal value of the electrical signal to be converted. At the output of the differential amplifier a differential signal representing the error in the conversion is obtained and is used to control the amplification of the electrical signal to be converted. These known circuits have a particular disadvantage: the feedback loop is supplied by a back facet laser signal which may deviate greatly from the signal of the main beam. This deviation in turn causes a distortion in the conversion.
An object of the present invention therefore is to develop a circuit for converting an electrical input signal into an optical output signal largely without distortion over a preferably wide frequency range. One approach in achieving this object is set forth in claim 1. Therein, a control beam is decoupled from a main beam generated by a light-producing element like a laser diode and is detected with a light-detecting element like a photodiode. The control beam is decoupled by means of a reflection with a transparent mirror or a glass pane. The intensity of the control beam lies within a few percent of the intensity of the main beam, for example, 4%. In this instance then, 96% of the main beam penetrates the mirror or glass pane and is available as a signal beam. A detector photodiode in response to the detected control beam produces an electrical "real" signal. The electrical real signal is coupled, through linear elements only, with a "nominal" signal having the nominal value of the signal to be converted to produce an error signal. The error signal is superimposed on the electrical signal to be converted at the input of a converter containing an error amplifier.
Preferably the input port of the converter is connected to the anode of the photodiode via a delay line and an ohmic resistor in series therewith; the cathode of the photodiode is connected to a positive direct current (d.c.) voltage source. The error signal is obtained at the node between the photodiode and the ohmic resistor.
In a converter capable of adjusting the optical output signal, the electrical input signal is applied to the inverted input port of a differential amplifier functioning as a controllable amplifier. Preferably, the amplifier is a wideband d.c. amplifier. The non-inverted input port serves as a control input and is connected to the output port of an error amplifier. The error amplifier, which corrects the laser control current, generates a corresponding control signal at its output in response to an error signal. The error amplifier may have a bandwidth considerably lower than that of the controllable amplifier. Thus, at frequencies outside the bandwidth of the error amplifier, the converter system operates in a noncontrolled, direct manner where only an average value of the the error signals is monitored and used. Specifically, a d.c. voltage is supplied to the control input port of the controllable amplifier, e.g., at the non-inverted input port, and the electrical input signal is applied to the inverted input port. The amplifier bandwidth of the controllable amplifier, which is connected in series with the laser diode, is not affected by the feedback loop.
A particularly advantageous embodiment of the invention includes a controllable amplifier having a gain control device with an adjustable gain for its output. This control device can be, for example, a multiplier circuit that is part of the d.c. amplifier. The gain of the control device is regulated such that the varying conversion factors at high and low frequencies are compensated for one another. In this manner the electrical input signal is converted into an optical output signal with a substantially constant conversion factor over the entire frequency range.
The circuit in accordance with the preferred embodiment has the added advantage of a wide bandwidth at a stable operating point and high linearity at low and medium frequencies. Regulation of the gain control device is preferably achieved by means of a microprocessor. For this purpose, a test signal is applied to the input port of the circuit and a calibration error signal resulting-therefor therefrom is coupled to a microprocessor, where the calibration error signal is evaluated. The microprocessor then adjusts the gain of the gain control device according to a preceding error signal. This adjustment of the amplification is repeated for several sequential steps until the desired minimization of the error signal is reached. This error minimization is accomplished with the use of a microprocessor with known methods of interval nesting.
For decoupling the control beam, a reflective medium like a thin glass pane is placed in the beam path of the main beam. The glass pane should be oriented in such a way that the angle of beam incidence is close to vertical incidence; in any case, the angle should be less than 45 degrees. In order to avoid interferences, the side of the glass pane distal to the laser diode may be made nonreflecting. Also to avoid interferences, the opposing sides of the glass pane can be nonparallel.
It is known that the reflection on a glass pane or a mirror is dependent on the polarization of the incoming light beam. For a beam with varying polarization directions, such as a beam resulting from varying modulations of a laser diode, correspondingly varying reflection coefficients are engendered. This generally results in a disruptive nonlinear relationship between main beam and control beam. For small angles of incidence, however, deviations of the reflection coefficients are relatively small and therefore do not need to be considered in many applications. However, it is possible to provide a polarization filter between the laser diode and the reflecting glass pane in order to further avoid the disruptive effect mentioned.